1. Field of the Invention
The present invention relates to an image reading method for reading an image signal corresponding to image information from a solid-state detector in which the image information is recorded by irradiation of a recording electromagnetic wave, and to an image recording and reading device using the foregoing solid-state detector.
2. Description of the Related Art
Nowadays, in radiography for the purpose of medical diagnosis and the like, a radiation image recording and reading device is known, which uses a radiation solid-state detector for detecting radiation and outputting an image signal representing radiation image information. Various types of detectors are proposed and put to practical use as the detector used in the above device.
From the aspect of a charge generation process for converting radiation into electric charges, for example, there are radiation solid-state detectors of the following types, including: a radiation solid-state detector of a photoconversion type, which temporarily accumulates signal charges in a charge storage section of a photoelectric conversion element, converts the accumulated charges into an image signal (an electrical signal) and outputs the image signal, the signal charges being obtained by detecting fluorescence emitted from a phosphor upon exposure to radiation by the photoelectric conversion element (for example, U.S. Pat. No. 4,803,359 and Japanese Unexamined Patent Publication No. 2 (1990)-164067, PCT International Publication Number W092/06501, SPIE Vol. 1443 Medical Imaging V; Image Physics (1991), pp. 108–119 and the like); a radiation solid-state detector of a direct conversion type, which temporarily accumulates signal charges in a charge storage section, converts the accumulated charges into an electrical signal and outputs the electrical signal, the signal charges being generated in a radiation conductor upon exposure to radiation and being collected by a charge collection electrode (MATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICONRADIATION DETECTORS, Lawrence Berkeley L. University of California, Berkeley. Calif. 94720 Xerox Parc. Palo Alto. Calif. 94304, Metal/Amorphous Silicon Multilayer Radiation Detectors, IEE TRANSACTIONS ON NUCLEAR SCIENCE. VOL. 36, NO. 2, APRIL 1989, Japanese Unexamined Patent Publication No. 1 (1989)-216290, and the like); and the like.
Moreover, from the aspect of a charge read-out process for reading out the accumulated charges, there are radiation solid-state detectors of the following types, including: a radiation solid-state detector of a TFT read-out type, which reads out the accumulated charges by performing scan drive of a TFT (thin film transistor) connected to the charge storage section; a radiation solid-state detector of an optical read-out type, which reads out the accumulated charges by irradiating a reading light (reading electromagnetic waves) on the detector; and the like.
Furthermore, there have been proposed radiation solid-state detectors of an improved direct conversion type in U.S. Pat. Nos. 6,268,614 and 6,376,857. The radiation solid-state detector of the improved direct conversion type is one of the direct conversion type and the optical read-out type. The radiation solid-state detector of the improved direct conversion type is constituted by sequentially laminating the following layers, including: a first electrode layer having transmissivity to recording radiation; a photoconductive layer for recording (hereinafter referred to as a recording photoconductive layer), which shows photoconductivity (to be accurate, radioconductivity) by receiving irradiation of the recording radiation transmitted through the first electrode layer; a charge transfer layer acting as substantially an insulator on charges of the same polarity as charges in the first electrode layer and acting as substantially a conductor for charges of a polarity opposite to that of the foregoing charges; a photoconductive layer for reading (hereinafter referred to as a reading photoconductive layer), which shows photoconductivity (to be accurate, electromagnetic conductivity) upon receipt of irradiation of an electromagnetic wave for reading; and a second electrode layer having transmissivity to the electromagnetic wave for reading. In the radiation solid-state detector of the improved direct conversion type, signal charges (latent image charges) carrying image information are accumulated on an interface (a charge storage section) between the recording photoconductive layer and the charge transfer layer.
Moreover, in the optical read-out type including the above improved direct conversion type, as a method for reading out the signal charged accumulated in the charge storage section, there are three methods, including, for example; a method for detecting the signal charges in such a manner that the second electrode layer (hereinafter referred to as a read electrode), that is an electrode on a side where a reading light is irradiated, is set to have a flat-plate shape and the read electrode is scanned with a spot-shaped reading light such as a laser; a method for detecting the signal charges in such a manner that the read electrode is set as a stripe electrode composed of arrays of a number of linear electrodes, a direction substantially at right angle to a longitudinal direction of the stripe electrode, that is, a longitudinal direction of each linear electrode is set to be a main scanning direction, the longitudinal direction is set to be a sub-scanning direction, and scanning with the spot-shaped reading light is performed in the main scanning and sub-scanning directions; and a method for detecting the signal charges by scanning with a line light source in the longitudinal direction (that is, the sub-scanning direction) of the stripe electrode, the line light source extending in the main scanning direction.
Moreover, in the radiation solid-state detector as described above, in reading out the charges accumulated in the charge storage section, since the foregoing radiation solid-state detector utilizes a semiconductor, not only a current corresponding to an amount of the charges accumulated in the charge storage section but a dark current is read out. Due to this dark current, a stationary gain or offset is included in an image signal outputted from the radiation solid-state detector, and thus a reproduced image based on an accurate image signal cannot be obtained. In addition, there a problem arises in that deterioration in an image quality of the reproduced image is caused. Moreover, in some cases, the dark current changes in accordance with temperature or changes over time. In order to avoid the above problems, there has been proposed a method for correcting the foregoing offset and gain by subtracting an image signal read out from a radiation solid-state detector with no radiation image information recorded therein from an image signal read out from a radiation solid-state detector with radiation image information recorded therein in Japanese Unexamined Patent Publication No. 7(1995)-072256. However, in this case, it is necessary to read the radiation solid-state detector with no radiation image information recorded therein, thus taking time and trouble. Moreover, a method for correcting the foregoing offset and gain without reading the radiation detector with no radiation image information recorded therein as described above has been proposed in Japanese Unexamined Patent Publication No. 2000-174982 and U.S. Pat. No. 6,333,505. Specifically, the above method is realized in such a manner that an insensitive pixel detecting no radiation is provided and the foregoing offset and gain are corrected by using a pixel signal corresponding to this insensitive pixel.
However, in Japanese Unexamined Patent Publication Nos. 2000-174982 and 2000-224377, the insensitive pixel is provided in a region other than a region where the radiation image information in the radiation solid-state detector is recorded, or is provided in an edge of the radiation solid-state detector. Thus, when the insensitive pixel is away from pixels in which the radiation image information is recorded, it is highly unlikely that the same dark current flows between these pixels. Therefore, accurate correction of the foregoing offset and gain was difficult to perform.